This invention relates in general to AC power supplies and specifically to automatic voltage doubler circuits for such power supplies.
U.S. Pat. No. 4,665,323, entitled ELECTRONICALLY SWITCHABLE POWER SOURCE issued May 12, 1987 in the names of R. Russell, W. Salata and D. Stevens and assigned to Zenith Electronics Corporation, describes a DC power source that automatically adapts to either a 120 volt AC or a 240 volt AC line input to produce a single range of regulated output voltage. The patented circuit utilizes a plurality of comparators for determining when the circuit should switch to the voltage doubler mode. It includes a triac that is supplied with a DC trigger voltage for switching between the two operating modes.
In Ser. No. 101,117, filed 9/25/87, now U.S. Pat. No. 4,783,729, entitled AUTOMATIC VOLTAGE DOUBLER SWITCH, in the name of John Konopka and assigned to Zenith Electronics Corporation, a circuit arrangement for accomplishing substantially the same result as the patented circuit, but with significantly fewer parts, is described and claimed. That circuit utilizes a triac that is supplied with short duration, high energy trigger pulses from a relaxation oscillator for operating a power supply in its voltage doubler mode, with the oscillator being inhibited when the power supply is operated in the direct mode.
The present invention, while accomplishing the same result as the patented circuit and the circuit of the copending application, does so in a different manner with still fewer parts and includes a desirable response delay in the presence of line surges while operating in the doubler mode. A feature of the present invention circuit is use of one of the rectifiers in the supply bridge rectifier network to supply operating voltage for an SCR trigger that senses the AC line voltage to disable the triac drive when a predetermined line voltage is reached.